The present invention relates to systems and methods of reducing carbon dioxide emissions in a fluid catalytic cracking (FCC) unit.
The fluidized catalytic cracking of hydrocarbons is the mainstay process for the production of gasoline and light hydrocarbon products from heavy hydrocarbon charge stocks such as vacuum gas oils or residual feeds. Large hydrocarbon molecules associated with the heavy hydrocarbon feed are cracked to break the large hydrocarbon chains thereby producing lighter hydrocarbons. These lighter hydrocarbons are recovered as product and can be used directly or further processed to raise the octane barrel yield relative to the heavy hydrocarbon feed.
The basic equipment or apparatus for the fluidized catalytic cracking of hydrocarbons has been in existence since the early 1940's. The basic components of the FCC process include a reactor, a regenerator, and a catalyst stripper. The reactor includes a contact zone where the hydrocarbon feed is contacted with a particulate catalyst and a separation zone where product vapors from the cracking reaction are separated from the catalyst. Further product separation takes place in a catalyst stripper that receives catalyst from the separation zone and removes trapped hydrocarbons from the catalyst by counter-current contact with steam or another stripping medium.
The FCC process is carried out by contacting the starting material—generally vacuum gas oil, reduced crude, or another source of relatively high boiling hydrocarbons—with a catalyst made up of a finely divided or particulate solid material. The catalyst is transported like a fluid by passing gas or vapor through it at sufficient velocity to produce a desired regime of fluid transport. Contact of the oil with the fluidized material catalyzes the cracking reaction. The cracking reaction deposits coke on the catalyst. Coke is comprised of hydrogen and carbon and can include other materials in trace quantities such as sulfur and metals that enter the process with the starting material. Coke interferes with the catalytic activity of the catalyst by blocking active sites on the catalyst surface where the cracking reactions take place.
Catalyst is traditionally transferred from the stripper to a regenerator for purposes of removing the coke by oxidation with an oxygen-containing gas. An inventory of catalyst having a reduced coke content relative to the catalyst in the stripper, hereinafter referred to as regenerated catalyst, is collected for return to the reaction zone. Oxidizing the coke from the catalyst surface releases a large amount of heat, a portion of which escapes the regenerator with gaseous products of coke oxidation generally referred to as flue gas or synthesis gas depending on the choice of feed gas to the regenerator (i.e., air to the regenerator generates flue gas while an artificially created gas comprising oxygen and (1) steam, (2) carbon dioxide or (3) steam and carbon dioxide will generate synthesis gas). The balance of the heat leaves the regenerator with the regenerated catalyst. The fluidized catalyst is continuously circulated from the reaction zone to the regeneration zone and then again to the reaction zone. The fluidized catalyst, as well as providing a catalytic function, acts as a vehicle for the transfer of heat from zone to zone. Catalyst exiting the reaction zone is spoken of as being spent, i.e., partially deactivated by the deposition of coke upon the catalyst. Specific details of the various contact zones, regeneration zones, and stripping zones along with arrangements for conveying the catalyst between the various zones are well known to those skilled in the art.
Refining companies are under increased pressure to reduce CO2 emissions as a result of carbon tax legislation and other drivers such as a desire to demonstrate long-term sustainability. Roughly 15-25% of refinery CO2 emissions are caused by the burning of catalyst coke in the FCC regenerator. Thus, there is a need to provide a way to reduce the carbon dioxide emissions in a fluid catalytic cracking unit.
Carbon dioxide emissions in a FCC unit can be reduced by regenerating spent catalyst under gasification conditions. There are two competing problems that must be addressed when gasifying catalyst coke in a regenerator: (1) completing regeneration of the catalyst and (2) maximizing synthesis gas quality. To fully regenerate the spent catalyst, a high concentration of oxygen is required in relation to the coke on the catalyst. The problem is that excess oxygen will lead to combustion of the synthesis gas, lowering the quality of the synthesis gas product. Thus, there is a need to provide a solution in which catalyst is completely regenerated and the synthesis gas product is maximized.